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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g3163

Using the Hammond postulate with bond energies (kcal/mol: $C - H$ in methane 105, $H - F$ 136, $H - Br$ 88), which steps are expected to have EARLY (reactant-like) transition states?

ATwo methyl radicals combining to form ethane
B $F X^{∙} + CH X_{4} \to HF + CH X_{3} X^{∙}$
C $Br X^{∙} + CH X_{4} \to HBr + CH X_{3} X^{∙}$
DIonisation of $(CH X_{3}) X_{3} C - Br$ into a free ion pair in solution

Answer: A, B

Radical combination is highly exothermic with almost no barrier, so it is early. The

F X^{∙}

step has

Δ H = 105 - 136 = - 31

kcal/mol, strongly exothermic, so it is early. The

Br X^{∙}

step has

Δ H = 105 - 88 = + 17

, endothermic, so it is late. Ionisation is endothermic and late. Answer: 1 and 2.

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